The development of optical fiber networks as a major component in communication systems has driven the development of optical cross-connect switches to enable the full flexibility, safety, and maintenance of these systems. Communication channels which are connected via optical fiber networks use micro-actuated mirrors as optical switches in optical cross-connects, in order to switch individual optical fibers and to enable connection with other optical fibers. The mirror serves to deflect the light from one optical fiber into the other and the mirror is either in a “clear” position or in a “deflecting” position. The object of the technologies devoted to building optical cross-connects is to create systems for movement of these mirrors in individual and programmable fashion. Mirrors which are placed in either one of two positions, either “clear” or “deflecting”, are known as 2D or digital. These micro-actuated mirrors are used in free-space switching and make use of MEMS technology.
The main problem with developing commercialized systems based on motion of these mirrors is reliability of the actuated micro-mirrors and manufacturability of the actuated micro-mirrors so that they become a producible item for mass-production and operate properly for the required prolonged duration in the field. In optical cross-connects at the local network level, speed of switching is not the issue as long as it is done in less than 50 msec, but the other issues mentioned previously are more important.
There are two techniques for machining structures onto chips so as to become integrated with them, these are bulk micro-machining of crystalline silicon, and surface micro-machining of polysilicon. Bulk micro-machining is machining of silicon crystals to give them physical features and a three-dimensional form. Surface micro-machining involves application of various structural and sacrificial layers of materials and selective etching techniques to shape the micro-mechanical structures as needed.
The prior art of actuated micro-mirrors is reported in the open literature in publications such as IEEE Journal of “Selected Topics in Quantum Electronics”, special issue on Microoptoelectromechanical systems (MOEMS), Vol.5(1), January/February 1999. In particular, prior art is discussed in the paper on “Free-Space Micromachined Optical Switches for Optical Networking”, by L. Y. Lin, E. L. Goldstein, and R. W. Tkach (Pages 4-9 in the above journal) and the following papers on Free-Space Optical Switches by H. Toshiyoshi et al. (Pages 10-17 in the above journal), C. R. Giles et al. (Pages 18-25 in the above journal) and A. A. Yasseen et al. (Pages 26-32 in the above journal) and references therein.
Additional prior art is reviewed in the paper by R. S. Muller and K. Y. Lau, “Surface-Micro machined Microoptical Elements and Systems”, in Proceedings of the IEEE, August 1998, vol.86, No.8 “Special Issue on Integrated Sensors, Microactuators, and Microsystems”, pages 1705-1720. Other papers in the above journal issue summarize the background and basics pertinent to the application of actuated micro-mirrors to Free-Space Optical Switches.
An example of a prior art optical switch actuated micro-mirror is an AT&T design in which the switch mirror is actuated by a pushrod which causes motion of the mirror about a hinged joint. The mechanical forces acting on the pushrod and mirror cause a gradual weakening of the material from which the joint is made, during repeated cycles of compression and tension of the material. Furthermore, the polysilicon material is thin and therefore fragile. Moreover, because of internal stresses and surface morphology of polysilicon, the performance of the mirrors is not as good as required. Thus, while the advantages of manufacturing using polysilicon are available for construction of the hinged joint, the use of surface-micromachining technology introduces inherent limitations.
The prior art describes, in the paper by Toshiyoshi et al, Journal of Microelectromechanical Systems, Vol. 5, No. 4, December 1996, the use of a polysilicon construction of mirrors on a wafer. The actuation of these mirrors is achieved by electrostatic actuation with a relatively high voltage of approximately 160 volts.
The electrostatic actuation involves the application of a voltage between a pair of metal plates, in a fashion similar to a capacitor, and if one plate is movable by rotation on a torsion beam, ultimately with the right voltage applied, one plate will rotate toward the other. If the movable plate is restrained by the mechanical restoring torque of the torsion beam, the voltage required to draw the plates together will be affected by the spring constant. The pull-in “effect” is defined as a phenomenon in which the electrostatic forces overcome the mechanical forces when the plates are at a certain distance or angle apart, so that the plates will be pulled together. If the movement of the plates is in accordance with an angular displacement, then the movement is a torsional one and the electrostatic torque provides the actuation. The pull-in phenomenon is widely discussed in the literature. See for example the following references and references therein:    [1] S. D. Senturia, “Microsystems Design”, Kluwer Academic Press, 2000 {in press).    [2] L. J. Hornbeck, “Deformable-Mirror Spatial Light Modulators”, SPIE Critical Review Series, Vol. 1150, Spatial Light Modulators and Applications III, pp. 86-102, 1989.    [3] E. S. Hung and S. D. Senturia, “Extending the Travel Range of Analog-Tuned Electrostatic Actuators”, JMEMS, Vol. 8, No. 4, December 1999, pp. 497-505.
[4] P. M. Osterberg and S. D. Senturia, ‘M-TEST: A Test Chip for MEMS Material Property Measurement Using Electrostatically Actuated Test Structures’, JMEMS, Vol. 6, No. 2, June 1997, pp. 107-118.    [5] O. Degani, E. Socher, A Lipson, T. Leitner, D. J. Seter, S. Kaldor and Y. Nemirovsky, “Pull-In Study of an Electrostatic Torsion Microactuator”, JMEMS, Vol. 7, No. 4, December 1998, pp. 373-379.    [6] E. K Chan and R. W. Dutton, “Electrostatic Micromechanical Actuator with Extended range of Travel”, JMEMS, Vol. 9, No. 3, September 2000, pp. 321-328.
The axis of rotation of the mirrors, which is called the torsion beam is subject to breakage because of its being very thin. The compliance, or flexibility of the torsion bean, is related to its thickness, so that if a stronger torsion beam is desired, a relatively high electrostatic actuation votage is needed, and this is problematic since it may cause reliability issues. Very high voltages may even cause dielectric breakdown of the electric field in the micro space environment. Some of the problems associated with the use of polysilicon mirrors are reported in the paper. Altogether, the polysilicon mirrors are fragile and may be stressed, thus degrading optical performance. The introduction of high voltage also introduces reliability issues.
Therefore, it would be desirable to provide an optical switch actuated micro-mirror, which features high reliability, and good manufacturability to insure an inexpensive solution for switching of fiber-optic networks in optical cross-connect switches.